@John P5 this is still a bit confusing and wanted to make sure there is no misunderstanding (or understand it myself).
You're saying this was 0 coupon but referring to accrued interest paid at acquisition, and interest paid to you.
T-Bills specifically, don't have accrued interest, your cost basis is what you paid in the market, and should not need adjustment.
If it was a zero-coupon Strips security, you may get an 1099-OID and your broker will adjust your cost basis for the accretion. Upon disposal of a discounted security thru sale or redemption, generally your 1099-B should reflect any remaining AMD in Box 1f and possibly also gain/loss relative to the AMD. AMD is taxable as ordinary income and will transfer to your Schedule B.
Are you able to clarify what instrument this was (CUSIP/description), whether you held to maturity or sell before maturity? What 'adjustment' are you making to the cost basis and where did you get that figure?
