Is it permissible to pay money instead of fasting for an expiation of an oath, after the intention to fast has been made but before commencing it, especially since the money is now available, and what is the required amount to be paid?

The reason for the expiation of an oath (kaffarat al-yameen) is breaking the oath. If someone is unable to fulfill the optional choices for expiation and fasts for three days, then later becomes able, their obligation is fulfilled. Some scholars permit giving money as the value of the food, and this value differs from one place to another. As for the expiation for delaying make-up fasting (kaffarat ta'kheer al-qada'), its reason is delaying the make-up of Ramadan (definition) fasting without a valid excuse. It consists of a "mudd" of food (approximately 750 grams of rice) for each day of delay. Feeding the poor does not substitute for fasting except in specific cases. It is recommended to give the expiation concurrently with or after making up the missed fasts.