That's not how it works. The time to decide how much you are willing to pay is before you bid, not after. A bid cannot be amended to reduce it's hidden maximum except in one special circumstance (dealing with the seller lowering an unmet Reserve Price) and in that situation it is done automatically by eBay.
You may ask the seller to cancel your bid but s/he is under no obligation to do so nor to respond to your request, and you don't have a valid reason to force it. You are obligated to pay the final price (plus s/h etc.) if you win.
