In my mind I said that if I am successful in a certain contract, I shall give R1000 every month to the poor.

Answer

Although technically in terms of the dry legal law of the Shariah, the vow has not been enacted, morally you are obliged to fulfil your pledge despite not having made it verbally. In your mind you spoke with Allah Ta'ala. Do not allow bukhl (stinginess) to eliminate the barkat from your contract. It is improper to renege on even an intention without valid reason.