fix: Transfer excess back and reset allowance

contracts/adapters/paraswap/BaseParaSwapBuyAdapter.sol

@@ -39,6 +39,7 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
    * @param maxAmountToSwap Max amount to be swapped
    * @param amountToReceive Amount to be received from the swap
    * @return amountSold The amount sold during the swap
+   * @return amountBought The amount bought during the swap
    */
   function _buyOnParaSwap(
     uint256 toAmountOffset,
@@ -47,7 +48,7 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
     IERC20Detailed assetToSwapTo,
     uint256 maxAmountToSwap,
     uint256 amountToReceive
-  ) internal returns (uint256 amountSold) {
+  ) internal returns (uint256 amountSold, uint256 amountBought) {
     (bytes memory buyCalldata, IParaSwapAugustus augustus) = abi.decode(
       paraswapData,
       (bytes, IParaSwapAugustus)
@@ -75,7 +76,6 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
     uint256 balanceBeforeAssetTo = assetToSwapTo.balanceOf(address(this));
 
     address tokenTransferProxy = augustus.getTokenTransferProxy();
-    assetToSwapFrom.safeApprove(tokenTransferProxy, 0);
     assetToSwapFrom.safeApprove(tokenTransferProxy, maxAmountToSwap);
 
     if (toAmountOffset != 0) {
@@ -101,12 +101,15 @@ abstract contract BaseParaSwapBuyAdapter is BaseParaSwapAdapter {
       }
     }
 
+    // Reset allowance
+    assetToSwapFrom.safeApprove(tokenTransferProxy, 0);
+
     uint256 balanceAfterAssetFrom = assetToSwapFrom.balanceOf(address(this));
     amountSold = balanceBeforeAssetFrom - balanceAfterAssetFrom;
     require(amountSold <= maxAmountToSwap, 'WRONG_BALANCE_AFTER_SWAP');
-    uint256 amountReceived = assetToSwapTo.balanceOf(address(this)).sub(balanceBeforeAssetTo);
-    require(amountReceived >= amountToReceive, 'INSUFFICIENT_AMOUNT_RECEIVED');
+    amountBought = assetToSwapTo.balanceOf(address(this)).sub(balanceBeforeAssetTo);
+    require(amountBought >= amountToReceive, 'INSUFFICIENT_AMOUNT_RECEIVED');
 
-    emit Bought(address(assetToSwapFrom), address(assetToSwapTo), amountSold, amountReceived);
+    emit Bought(address(assetToSwapFrom), address(assetToSwapTo), amountSold, amountBought);
   }
 }

contracts/adapters/paraswap/BaseParaSwapSellAdapter.sol

@@ -98,6 +98,7 @@ abstract contract BaseParaSwapSellAdapter is BaseParaSwapAdapter {
         revert(0, returndatasize())
       }
     }
+
     require(
       assetToSwapFrom.balanceOf(address(this)) == balanceBeforeAssetFrom - amountToSwap,
       'WRONG_BALANCE_AFTER_SWAP'

contracts/adapters/paraswap/ParaSwapRepayAdapter.sol

@@ -114,7 +114,7 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
     // Pull aTokens from user
     _pullATokenAndWithdraw(address(collateralAsset), msg.sender, collateralAmount, permitSignature);
     //buy debt asset using collateral asset
-    uint256 amountSold = _buyOnParaSwap(
+    (uint256 amountSold, uint256 amountBought) = _buyOnParaSwap(
       buyAllBalanceOffset,
       paraswapData,
       collateralAsset,
@@ -127,15 +127,23 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
 
     //deposit collateral back in the pool, if left after the swap(buy)
     if (collateralBalanceLeft > 0) {
-      IERC20(collateralAsset).safeApprove(address(POOL), 0);
       IERC20(collateralAsset).safeApprove(address(POOL), collateralBalanceLeft);
       POOL.deposit(address(collateralAsset), collateralBalanceLeft, msg.sender, 0);
+      IERC20(collateralAsset).safeApprove(address(POOL), 0);
     }
 
     // Repay debt. Approves 0 first to comply with tokens that implement the anti frontrunning approval fix
-    IERC20(debtAsset).safeApprove(address(POOL), 0);
     IERC20(debtAsset).safeApprove(address(POOL), debtRepayAmount);
     POOL.repay(address(debtAsset), debtRepayAmount, debtRateMode, msg.sender);
+    IERC20(debtAsset).safeApprove(address(POOL), 0);
+
+    {
+      //transfer excess of debtAsset back to the user, if any
+      uint256 debtAssetExcess = amountBought - debtRepayAmount;
+      if (debtAssetExcess > 0) {
+        IERC20(debtAsset).safeTransfer(msg.sender, debtAssetExcess);
+      }
+    }
   }
 
   /**
@@ -170,7 +178,7 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
       initiator
     );
 
-    uint256 amountSold = _buyOnParaSwap(
+    (uint256 amountSold, uint256 amountBought) = _buyOnParaSwap(
       buyAllBalanceOffset,
       paraswapData,
       collateralAsset,
@@ -180,9 +188,9 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
     );
 
     // Repay debt. Approves for 0 first to comply with tokens that implement the anti frontrunning approval fix.
-    IERC20(debtAsset).safeApprove(address(POOL), 0);
     IERC20(debtAsset).safeApprove(address(POOL), debtRepayAmount);
     POOL.repay(address(debtAsset), debtRepayAmount, rateMode, initiator);
+    IERC20(debtAsset).safeApprove(address(POOL), 0);
 
     uint256 neededForFlashLoanRepay = amountSold.add(premium);
 
@@ -194,6 +202,14 @@ contract ParaSwapRepayAdapter is BaseParaSwapBuyAdapter, ReentrancyGuard {
       permitSignature
     );
 
+    {
+      //transfer excess of debtAsset back to the user, if any
+      uint256 debtAssetExcess = amountBought - debtRepayAmount;
+      if (debtAssetExcess > 0) {
+        IERC20(debtAsset).safeTransfer(initiator, debtAssetExcess);
+      }
+    }
+
     // Repay flashloan. Approves for 0 first to comply with tokens that implement the anti frontrunning approval fix.
     IERC20(collateralAsset).safeApprove(address(POOL), 0);
     IERC20(collateralAsset).safeApprove(address(POOL), collateralAmount.add(premium));

contracts/mocks/swap/MockParaSwapAugustus.sol

@@ -20,6 +20,9 @@ contract MockParaSwapAugustus is IParaSwapAugustus {
   uint256 _expectedToAmountMax;
   uint256 _expectedToAmountMin;
 
+  uint256 _excessFromAmount;
+  uint256 _excessToAmount;
+
   constructor() {
     TOKEN_TRANSFER_PROXY = new MockParaSwapTokenTransferProxy();
   }
@@ -58,6 +61,11 @@ contract MockParaSwapAugustus is IParaSwapAugustus {
     _expectedToAmountMax = toAmountMax;
   }
 
+  function expectExcess(uint256 excessFrom, uint256 excessTo) external {
+    _excessFromAmount = excessFrom;
+    _excessToAmount = excessTo;
+  }
+
   function swap(
     address fromToken,
     address toToken,
@@ -72,10 +80,17 @@ contract MockParaSwapAugustus is IParaSwapAugustus {
       'From amount out of range'
     );
     require(_receivedAmount >= toAmount, 'Received amount of tokens are less than expected');
-    TOKEN_TRANSFER_PROXY.transferFrom(fromToken, msg.sender, address(this), fromAmount);
-    MintableERC20(toToken).mint(_receivedAmount);
-    IERC20(toToken).transfer(msg.sender, _receivedAmount);
+    TOKEN_TRANSFER_PROXY.transferFrom(
+      fromToken,
+      msg.sender,
+      address(this),
+      fromAmount - _excessFromAmount
+    );
+    MintableERC20(toToken).mint(_receivedAmount + _excessToAmount);
+    IERC20(toToken).transfer(msg.sender, _receivedAmount + _excessToAmount);
     _expectingSwap = false;
+    _excessFromAmount = 0;
+    _excessToAmount = 0;
     return _receivedAmount;
   }
 
@@ -93,10 +108,17 @@ contract MockParaSwapAugustus is IParaSwapAugustus {
       'To amount out of range'
     );
     require(_fromAmount <= fromAmount, 'From amount of tokens are higher than expected');
-    TOKEN_TRANSFER_PROXY.transferFrom(fromToken, msg.sender, address(this), _fromAmount);
-    MintableERC20(toToken).mint(toAmount);
-    IERC20(toToken).transfer(msg.sender, toAmount);
+    TOKEN_TRANSFER_PROXY.transferFrom(
+      fromToken,
+      msg.sender,
+      address(this),
+      _fromAmount - _excessFromAmount
+    );
+    MintableERC20(toToken).mint(toAmount + _excessToAmount);
+    IERC20(toToken).transfer(msg.sender, toAmount + _excessToAmount);
     _expectingSwap = false;
+    _excessFromAmount = 0;
+    _excessToAmount = 0;
     return fromAmount;
   }
 }