Best away to keep monthly records of loans
Real world financial transactions do not follow simple assumptions such as "the client will make one payment per month".

Make a TRANSACTION table that records money coming in and money going out along with the details of that transaction, such as where the money came from (which client), where the money went to (which loan), and importantly when this happened (the date).  You might want to consider adding a transaction type to provide extra information, with values like "loan", "payment", "adjustment" etc.