You do not "pay the IRS" to get a higher pension. Are you referring to getting Social Security benefits---since you mentioned having "40 credits". 40 credits is the minimum you must have in order to be eligible to receive Social Security benefits. You are certainly allowed to continue working and get more than 40 credits and continue to pay in to Social Security in order to get a higher amount when you finally retire from working. If you are living abroad and are not paying in to Social Security any more, then your benefits will be based on the amount you worked for and paid into the system. You should check with Social Security to see what your SS benefits are if you retire now at 63 or wait and retire at a later time.
https://www.ssa.gov/agency/contact/