Add validation for negative fee amount

asserter/block.go

@@ -320,7 +320,20 @@ func (a *Asserter) Operations( // nolint:gocognit
 					)
 				}
 
-				val, _ := new(big.Int).SetString(op.Amount.Value, 10)
+				val, err := types.BigInt(op.Amount.Value)
+				if err != nil {
+					return err
+				}
+
+				// Validate that fee operation amount is negative
+				if val.Sign() != -1 {
+					return fmt.Errorf(
+						"%w: operation index %d",
+						ErrFeeAmountNotNegative,
+						op.OperationIdentifier.Index,
+					)
+				}
+
 				feeTotal.Add(feeTotal, val)
 				feeCount++
 			}

asserter/block_test.go

@@ -293,6 +293,14 @@ func TestOperationsValidations(t *testing.T) {
 			},
 		}
 
+		invalidFeeAmount = &types.Amount{
+			Value: "100",
+			Currency: &types.Currency{
+				Symbol:   "BTC",
+				Decimals: 8,
+			},
+		}
+
 		validAccount = &types.AccountIdentifier{
 			Address: "test",
 		}
@@ -513,6 +521,88 @@ func TestOperationsValidations(t *testing.T) {
 			construction:       false,
 			err:                ErrRelatedOperationInFeeNotAllowed,
 		},
+
+		"fee amount is non-negative": {
+			operations: []*types.Operation{
+				{
+					OperationIdentifier: &types.OperationIdentifier{
+						Index: int64(0),
+					},
+					Type:    "PAYMENT",
+					Status:  types.String("SUCCESS"),
+					Account: validAccount,
+					Amount:  validDepositAmount,
+				},
+				{
+					OperationIdentifier: &types.OperationIdentifier{
+						Index: int64(1),
+					},
+					Type:    "PAYMENT",
+					Status:  types.String("SUCCESS"),
+					Account: validAccount,
+					Amount: &types.Amount{
+						Value: "-2000",
+						Currency: &types.Currency{
+							Symbol:   "BTC",
+							Decimals: 8,
+						},
+					},
+				},
+				{
+					OperationIdentifier: &types.OperationIdentifier{
+						Index: int64(2),
+					},
+					Type:    "FEE",
+					Status:  types.String("SUCCESS"),
+					Account: validAccount,
+					Amount:  invalidFeeAmount,
+				},
+			},
+			validationFilePath: "data/validation_fee_and_payment_unbalanced.json",
+			construction:       false,
+			err:                ErrFeeAmountNotNegative,
+		},
+
+		"fee amount is negative as expected": {
+			operations: []*types.Operation{
+				{
+					OperationIdentifier: &types.OperationIdentifier{
+						Index: int64(0),
+					},
+					Type:    "PAYMENT",
+					Status:  types.String("SUCCESS"),
+					Account: validAccount,
+					Amount:  validDepositAmount,
+				},
+				{
+					OperationIdentifier: &types.OperationIdentifier{
+						Index: int64(1),
+					},
+					Type:    "PAYMENT",
+					Status:  types.String("SUCCESS"),
+					Account: validAccount,
+					Amount: &types.Amount{
+						Value: "-2000",
+						Currency: &types.Currency{
+							Symbol:   "BTC",
+							Decimals: 8,
+						},
+					},
+				},
+				{
+					OperationIdentifier: &types.OperationIdentifier{
+						Index: int64(2),
+					},
+					Type:    "FEE",
+					Status:  types.String("SUCCESS"),
+					Account: validAccount,
+					Amount:  validFeeAmount,
+				},
+			},
+			validationFilePath: "data/validation_fee_and_payment_unbalanced.json",
+			construction:       false,
+			err:                nil,
+		},
 	}
 
 	for name, test := range tests {

asserter/errors.go

@@ -103,6 +103,7 @@ var (
 	ErrFeeAmountNotBalancing       = errors.New("fee amount doesn't balance")
 	ErrPaymentCountMismatch        = errors.New("payment count doesn't match")
 	ErrFeeCountMismatch            = errors.New("fee count doesn't match")
+	ErrFeeAmountNotNegative        = errors.New("fee amount is not negative")
 
 	BlockErrs = []error{
 		ErrAmountValueMissing,
@@ -142,6 +143,7 @@ var (
 		ErrDuplicateRelatedTransaction,
 		ErrPaymentAmountNotBalancing,
 		ErrFeeAmountNotBalancing,
+		ErrFeeAmountNotNegative,
 	}
 )