Hi There. One month ago, I hit the back wheel of a girl on a racing bike whilst on the roundabout. Due to sun glare, I did not see her. Several conversations were had and we both agreed we would not report it as no major injury was suffered. However, as her bike was a racing bike, the small damaged cost quite a bit to repair. She offered to divide the payment of the repairs in half and stated this in written form (Facebook inbox). When she took her bike into get fixed, major repairs cost $610 and her father paid $550 extra to get another part fixed for security. She stated that "I would not have to pay for this extra $550 and halving the cost it should be around $300 each". Yesterday, she came around to settle the payment. I had written up a settlement agreement for both of us to sign in order to have written proof there was a meeting of the minds. (I gave her time to read and stated that she could make amendments where necessary). We both were happy and I ended up paying more than half ($405). We both signed the contract (one copy for each of us) and sent her my receipt for the payment. Half an hour after this contract had been signed, she messaged me stating that I needed to pay more and I needed to pay the full $610. She then proceeded to say the total amount was $1160 ($550 +610), in which i needed to pay half $580. She then continued to change the price back to the $610 that i was to pay. As we both have written copies of the agreement and Messages stating that I am only liable to pay half of the $610, do I have a right not to pay her anymore money under Contract Law even though the accident was my fault? Thank you in advance.