en_tn_lite_do_not_use/1co/15/10.md

his grace in me was not in vain

"because he was kind to me, I was able to do much good work"

the grace of God that is with me

Possible meanings are 1) this is literally true, and God actually did the work and kindly used Paul as a tool or 2) Paul is using a metaphor and saying that God was kind to let Paul do the work and to make Paul's work have good results.