1. When the diff amount is for "combine" bank payment method, the
account.move record used to represent the combine amount is modified
to contain the diff amount. The journal entry is modified:

FROM:
```
                Debit                           Credit
Outstanding     combine_amount
Receivable                                      combine_amount
```

TO:
```
		Debit				Credit
Outstanding 	combine_amount - diff_amount
Receivable					combine_amount
Diff account	diff_amount
```

2. When the diff amount is for "split" bank payment method, an
account.move moving the diff amount from Outstanding to Diff account
is created. The journal entry is like:

```
                Debit                           Credit
Outstanding     diff_amount
Diff account    				diff_amount
```

closes odoo/odoo#77859

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>