IIRC, the 240 ABS system used the speed sensor on the differential as input, and if so, the slight difference in individual rear wheel RPM's with mismatched tire sizes should not matter. I believe this was called 3-Channel ABS, whereas 4-channel ABS monitors each wheel separately.

The way to be certain on your car is to look at the rear wheel brake drum/disc assemblies and see if there is a tone ring (toothed wheel) attached to each, with a little sensor and wiring close to it.

If not, it shouldn't upset the ABS with mismatched rear tires. As Michael Yount said, it's not advisable to go a long distance with that setup, as the diff gears are "working" all the time, instead of only when rounding a curve.

Finally, the 245 wagons used somewhat larger tires than the sedans...did Volvo supply a correspondingly larger spare tire to exactly match the OE wagon tires? If not, I would think they found that small differences in tire size do not cause any ABS confusion.
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Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), and XC60. Also '77 MGB and four old motorcycles. Long gone: 1981 244, 1994 940 and 1998 S90.