וַיֹּ֕אמֶר אָנֹכִ֛י אֲשַׁלַּ֥ח גְּדִֽי־עִזִּ֖ים מִן־הַצֹּ֑אן וַתֹּ֕אמֶר אִם־תִּתֵּ֥ן עֵרָב֖וֹן עַ֥ד שָׁלְחֶֽךָ׃
猶大說：「我會從羊群中取一隻山羊羔送來給你。」她說：「你若給我一件抵押品，直到你送來為止，〔我就依從你〕。」
Rashbam
עד שלחך. The reason why we have the “weak” mode, i.e. the conjugation kal instead of the transitive conjugation hiphil, is to indicate that Yehudah would send a messenger to deliver the kid to Tamar. We find the use of the conjugation kal in connection with the sending of gifts especially, as for instance in 32,19 where Yaakov had instructed his messengers to Esau to describe the gift in such terms. (מנחה היא שלוחה) We find it again in verse 23 of our chapter where Yehudah says הנה שלחתי הגדי הזה, “here, I have sent this kid, etc.”
Sforno
אם תתן ערבון, if you are prepared to give me such tokens I am willing to sleep with you as you requested.