Translation:He said he would return the money tomorrow.
English is among the languages that have "tense agreement" in indirect speech. If the main clause is in past tense, verbs referring to the present need to be in past tense too (e.g. "would" instead of "will"). I know about this because I'm not a native speaker. (My native Hungarian works the same way in this regard as Japanese; they both use "natural sequence"). So technically speaking, "he said he will" is incorrect grammar in most prestige dialects of English.
No. This is wrong. In English there are situations where you are not required to change the tense, even being a grammar pedant. Feel free to check the link here, esp. section "No backshift": https://dictionary.cambridge.org/grammar/british-grammar/reported-speech/reported-speech-indirect-speech
Thanks for pointing this out! I wasn't aware of this exception where "We don’t need to change the tense in indirect speech if what a person said is still true or relevant or has not happened yet." That indeed seems to be the case for this particular example. However, the article also demonstrates that "will becomes would" is the general rule in reported speech, which works in all situations. So it seems that knowing about this exception would make you the grammar pedant, not me. ;)
It is true that they are different tenses, but will and would are no such thing in Japanese, or at least, not in this sentence. We would need context to know which it truly is, so without context、both exist in a state of being correct at the current moment, without context.
Something that continues to make this language complex for me is the phrasing of how it's spoken. For example, in this sentence, あしたお is said together, then a slight pause before finishing the word for money. I'm expecting each word to be spoken on its own, but it doesn't sound like they are, at least to me.
Another aspect that I'm hoping comes with experience is that I'm still not certain about when to drop certain sounds. As in this same case, I don't here a clear た sound followed by お; it sounds like と. It seems similar to English contractions yet without any change in the written form.