Frustration with exchange at identity store

I purchased a sweater with a R75 voucher and paid in the difference of R55.i mislaid the till slip and found it 2 months later to exchange for a bigger size.the cashier processed the transaction put R55 on the desk an told me to pay in R75 more but I was merely exchanging for a bigger size.store manager said because the voucher had already expired I had to repurchase at full price.this just did not make sense at all I asked for my original sweater back an they refused saying it belongs to identity.i walked out the store without taking the R55 as this was just not on.i emailed the head office and was told the same story.i returned again on 13/10/23 demanding my sweater and left the store again without it.if an item is purchased with a voucher so we cannot exchange at all? Someone please help solve this issue.thanks