They were still making the 924 in 1988? Yes, the 944 hadn't totally pushed its cheaper sibling aside as the 1980s drew to a close, though 1988 was the very last year for the 924. By this time, the old Audi engine was gone, replaced by the 944's Porsche-made engine (the 944S had a more powerful version of the engine).
Still, the '88 924S was a pretty good deal; at $19,900, it was $5,600 cheaper than the base 944, yet had the same 147 horses under the hood. This example seems fairly solid, though there's no telling how far it is from the dreaded timing belt replacement that keeps these cars inexpensive today.