A mistake made in store is now my problem having to finance R100.00 for a refund of R50.00

I bought 2 same cellphones both displayed at R849.00 but i was charged R849.00 and R899.00 and when i enquired i was told to come back to the store with a petrol of R100.00 for a refund of R50.00, this is very dissapointing as i have been a loyal customer for over 20yrs in Truworths.