No knowledge of what to do

I bought two fitted sheets from @home , when it was on sale around the 24th of January. On Sunday I opened the second sheet to find a flat sheet in the fitted sheet packaging. I went to the store to exchange it and was told I would need to pay the difference as it is no longer on sale?? Even though I just wanted the correct sheet. Obviously the people in the store have no idea how to manage exchanges. I then was sent to Busi who allowed the exchange. I then told them I would like to exchange the colour, which I was promptly told no. Even though it's in the 30 day exchange period and everything is in order. Apparently I can only exchange if the sale was on, or pay the difference? I asked for the policy , which they said they can't show me though that's what they are taught. I showed them the exchange policy on the receipt which they ignored. I have now read the full policy which does not state that. So why are staff lying? Also if they say something they should be able to produce the policy that stipulates that. All they kept saying is the till wont balance? Because of me exchanging the colour? Also how does the till affect me as a customer. @home needs to educate their staff!