Merge pull request #7876 from jniles/docs-add-prepayment-links

docs: add prepayment text to cash-payments

docs/en/finance/bookkeeping/cash-payments.md

@@ -62,3 +62,17 @@ In the above transaction, the cash account \(570001\) is **debited $10.00**, ind
 Some institutions accept _prepayments_.  These payment are made without reference to an invoice, in anticipation of future invoices made against the patient.
 
 As compared to _invoice payments_, creating a prepayment is simple.  The user must select the prepayment option and enter the amount received into the cash payment form.  The underlying transaction generated will contain two lines: one that debits the cash box and a second that credits the patient's account.
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+Importantly, if a prepayment exists on a patient's account during invoicing, a secondary transaction will be generated which allocates part or all of the prepayment amount to the invoice.  If the amount exceeds the invoice, then a remainder will exist on the prepayment and it may be used again.  If multiple prepayments exist, they may be combined to pay the total of the invoice as needed.  Notably, these transactions are made against *a single account*.  They serve not to move balances between accounts, but to track the allocation of the patient's funds from a credit with the institution to payment for the invoices.  
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+As an example, suppose that patient PA.HEV.1 has made two prepayments: the first for $5 USD (CP.TPA.2) and the second for $13 USD (CP.TPA.3).  Subsequently, an invoice is made for $20 USD (IV.TPA.3).  The following transaction will be made as a Journal Voucher immediately following the invoice:
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+
+| Transaction | Record | **Account** | Debit | Credit | Entity | Reference |
+| :--- | :--- | :--- | :--- | :--- | :--- | :--- |
+| TRANS2 | VO.TPA.1 | 410001 |        | $18.00 | PA.HEV.1 | IV.TPA.3 | 
+| TRANS2 | VO.TPA.1 | 410001 |  $5.00 |        | PA.HEV.1 | CP.TPA.2 |
+| TRANS2 | VO.TPA.1 | 410001 |  $13.0 |        | PA.HEV.1 | CP.TPA.3 |
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+After this transaction, the patient will have an outstanding balance of $2 USD on their invoice for the remaining amount.
+