Keep in mind the time in jump IS variable, so it's only ever a rough equivalent. As well as that, communications limited to the speed of travel probably means that deadlines are a bit flexible, and the odd late payment is expected.
If you really want to drill down to the nitty gritty of payments and not just abstract it, most likely you'd need a prime world that the mortgage is administered on, and an account held there from which the payments are deducted. Funds can be forwarded to there from a suitable port, but will take time to arrive and be validated, so planning is needed.
I generally just go with major banks having branches at A and B starports, and allow deposits to be made there (the information travels securely by mail to the network, but that does mean you may run in arrears until the balances catch up). Credit rating would have a great deal to do with how much leeway a person or business would get.
Further thought - the basic time unit SHOULD logically be the mean time for jump. Everything is going to depend on that; it's the interstellar "day".
As well, the "one week in jump, one week in port" may well be a legal requirement for most spacers' working conditions. Skipping shore leave entitlements may require negotiation, or double pay.