I understand why the Duolingo people made this error. The conditional in English used to be indicated by the subjunctive mood (as they have done here). However, English has changed over the centuries and the subjunctive now only exists in a few archaic sayings like "if I were you" (and even that is changing to "if I was you"). The correct English translation is If he/she buys potatoes.....
The whole Swahili program needs a major overhaul. The "correct" English translations become unacceptable.
Yes. Now, if they wanted "If he were to buy potatoes", it would at least be grammatical. This is the worst English translation I've found so far, and it does seem as if there is no one working on the course at the present time. I have reported lots of these and am getting nothing back, much like the guarani course that was abandoned (and seems to be reviving, since I have gotten a lot of your translation has been accepted notifications)
Duolingo you are wrong and we are right! It is: If he buys potatoes, he will pay.