Is it permissible to suffice with fasting as expiation for an oath if one was unable to feed the poor in the past, even if it is possible now, and should the expiation be based on the amount of money that was available at the time of the oath or the amount of money available now?

If you only possess one hundred riyals, it appears that you are unable to expiate except through fasting. Fasting for three days will suffice you in lieu of feeding, clothing, or freeing a slave, based on His (Allah's) saying: "But whoever finds none - then a fast of three days. That is the expiation for oaths when you have sworn. And guard your oaths. Thus does Allah make clear to you His verses that you may be grateful." (Al-Ma'idah: 89). Once expiation is completed by fasting due to inability, you are not required to expiate again when you become able to do so by means other than fasting.