It's of course a good idea to verify the part number of your existing ECU. If you haven't been there before, it's fairly easy to access. As Spook mentioned, it lives behind the right side panel in the passenger footwell. Start by removing the under dash panel, then lift the plastic sill trim (with a good yank straight up) and peel back the flex black edging starting at the bottom. A camera is ideal to take a pic of the label.

You don't mention if your '94 940 is an NA engine with Regina or a turbo with LH 2.4. If it's LH 2.4 then Bill Garland's ECU table is an excellent reference for part numbers http://www.nuceng.ca/bill/volvo/database/ecu.htm. The main thing with LH 2.4 ECUs is to match the EGR capability of your engine, which also determines whether a cold start injector is present, and as well matching turbo vs. non-turbo and whether there is Pulsair. The B230F NA (more properly a B230FD in later years) switched from Regina to LH 2.4 with Pulsair in 1995 (perhaps also the odd 1994 based on anectodal reporting I vaguely recall, possibly in different markets). EGR presence was normally market dependent, so if for instance you find a used LH 2.4 ECU in California or Canada, it's more likely EGR. Turbo 940s were always LH 2.4 without Pulsair, as was the early 940 B234F 16-valve.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now