My sibling made a verbal agreement with our father over the purchase of a truck. Our father took the money out of a mortgage loan and our father died before the total bill was paid. I have an avidavit by someone who witnessed the agreement. Is there anyway I, as executor of his will, can keep the sibling from inheritating any more of the estate? And how can I do this?

Does he agree he owes the money ? The estate can sue him for the money he owes for the truck, if you win, that amount may be held from the money.
You may want to pay him all the money except for that amount, place that amount in escrow pending outcome of the legal case