Claim of right credit is not $18k. Taxpayer paid tax on $18k in the year he/she claimed said income. The credit is the amount of tax they paid in that year due to that extra income. You may want to file your return without (assuming substantial refund) beforehand and then amend for claim of right. But thats your clients call. My client had a refund without claim of right and because of claim of right took IRS a full year to issue full refund. PITA.