لَئِنْ سَأَلْتَ فُلَانًا لَتَسْأَلَنَّ بِهِ الْبَحْرَ
If you ask that man for a gift, it is as though you will ask the sea through him.
The aim is to magnify his generosity. His bounty is so expansive that a “sea” appears to be drawn from him and encountered in his person.
The pronoun of “bihi” refers to the original man, so the bāʾ attaches to the source of extraction. “Al-baḥr” is the counterpart brought out for its associations of abundance.
Calling a man “a sea of generosity” may be metaphorical in a more direct fashion; this conditional sentence and bāʾ create the distinctive tajrīd structure.