Excel Formula - How to check in column, if true then use next columns value

I basically have a spreadsheet that calculates the remaining balance and adds payments with interest over time.

What I want to do is keep 1 tab for the calculations and one tab for payments.

So basically, when my Calculation tab which keeps adding dates comes across a row on a payment tab it adds that value to payments (column b). This is what I have but I know this is not robust enough.

=IF(A1276=Sheet2!A1,Sheet2!B1,0)

calc tab

payments tab