כִּֽי־תִדֹּ֥ר נֶ֙דֶר֙ לַיהוָ֣ה אֱלֹהֶ֔יךָ לֹ֥א תְאַחֵ֖ר לְשַׁלְּמ֑וֹ כִּֽי־דָּרֹ֨שׁ יִדְרְשֶׁ֜נּוּ יְהוָ֤ה אֱלֹהֶ֙יךָ֙ מֵֽעִמָּ֔ךְ וְהָיָ֥ה בְךָ֖ חֵֽטְא׃
אַז דו װעסט מנדר זײַן אַ נדר צו ה׳ דײַן גאָט, זאָלסטו זיך ניט שפּעטיקן אים צו באַצאָלן; װאָרום מאָנען װעט אים מאָנען ה׳ דײַן גאָט פֿון דיר, און אױף דיר װעט זײַן אַ זינד.
Rashi
לא תאחר לשלמו [WHEN THOU SHALT VOW A VOW UNTO THE LORD THY GOD] THOU SHALT NOT DELAY TO PAY IT — beyond three festivals after the vow has been made. Our Rabbis have deduced it (the fact that one does not transgress the prohibition before three festivals elapse) from a Scriptural text (Rosh Hashanah 4b).
Chizkuni
כי תדור נדר, “When you shall vow a vow, etc.” It is possible that this verse appears here as we read previously (verse 19) about a harlot not being allowed to fulfill a vow she had made to the Lord by using money earned from practicing her trade.
Ibn Ezra
Having mentioned the prohibition of “the earnings of a harlot … for any vow” [: 19], Scripture now discusses the case of when you vow a donation.