How can I correct the return amount for a non-discounted item?

Retail & Point of Sale
1

i did an offer like this:

from certain tag - buy 2 for 100USD

all other shoes costs 69

bought 3 pair of shoes - 2 from the certain tag for 100USD and one not.

all costs 169

the offer i manage with PIXEL UNION app.

now i want to do a return for only one pair. the pair with no discount.

it should return me 69/ but it calculates 169 divide to 3 and returns only 56 USD

how can i fix the problem?