@Eldwin though I would personally be much more satisfied if he is disappeared or even publicly made an example of. Would fit the times better.
Although for the latter, there is an option that doesnt fit the times well, but does fit for a modern isekai protagonist - making a public display of him by setting him free saying they did a miscalculation with the horses anyway and needed to get rid of one or get another horse (and shooing him out of the army, maybe helping him find another job for PR).
Even if it was a lie, it can easily be made truth by making one of the horses be used by his high-ranked men (not only cavalry needs horses) or gifting it to logistics (always need more of that after all)
@Iamnormal
You're pretty close to the best solution to the problem as well. Since he's severely impugned your honor by suggesting your word is not to be trusted (i.e. that you are a dishonorable cur) you need to duel or otherwise beat him up - but in a fair and obviously honorable way. (Since he's currently a slave, dick-punching is a valid option for sure, but since the optimal end result is still that he join your cavalry forces it might be better to go a bit easy on the permanent or scummy damages)
@Luftwaffles27 their footmen would look like Swiss pikemen:
Those guys were famous on the Middle Ages for being able to fend off bigger armies, using terrain for their advantage. Here's an example: https://en.wikipedia.org/wiki/Battle_of_Laupen