סימן סה סעיף כג
מֻתָּר לְהַרְבּוֹת בִּנְדוּנְיַת חֲתָנִים, כְּגוֹן שֶׁפָּסַק נְדוּנְיָא לְבִתּוֹ וְהִתְנָה עִם חֲתָנוֹ שֶׁכָּל שָׁנָה שֶׁיַּנִּיחַ אֶצְלוֹ אֶת הַנְּדוּנְיָא יִתֵּן לוֹ כָּךְ וְכָךְ שָׂכָר, מֻתָּר, שֶׁאֵין זֶה אֶלָּא כְּמוֹסִיף לוֹ נְדוּנְיָא, וּכְאִלּוּ אָמַר לוֹ אֲנִי נוֹתֵן לְךָ מַתָּנָה כָּךְ וְכָךְ לִזְמַן פְּלוֹנִי. וְאִם לֹא אֶתֵּן לְךָ לִזְמַן פְּלוֹנִי, עוֹד אֲנִי מוֹסִיף לְךָ כָּךְ וְכָךְ, דְּמֻתָּר. וְדַוְקָא כְּשֶׁהִתְנוּ כֵן מִיָּד בִּשְׁעַת כְּתִיבַת הַתְּנָאִים. דְּכֵיוָן דְּעַד עַתָּה לֹא הָיָה עָלָיו שׁוּם חִיּוּב, אִם כֵּן הַכֹּל הוּא חִיּוּב אֶחָד. אֲבָל אִם בִּשְׁעַת כְּתִיבַת הַתְּנָאִים, נִתְחַיֵּב בִּסְתָם סַךְ נְדוּנְיָא וּבִשְׁעַת הַחֲתֻנָּה רוֹצִים לְהִתְפַּשֵּׁר לָתֵת לוֹ דְּבַר מַה בִּשְׁבִיל הַרְחָבַת הַזְּמָן, אָסוּר, וּצְרִיכִין לַעֲשׂוֹת בְּדֶרֶךְ הֶתֵּר.
It is permitted to increase the dowries of bridegrooms. For example, if one set aside a dowry for his daughter and agreed with his son-in-law that for every year he leaves the dowry with him, he will give him so much profit, this is permitted; for this is considered only as an increase in the dowry. It is as if he had said to him: "I will give you a gift of so much at a certain time, and if I fail to give it to you by that time, I will add to it so much and so much," which is permitted. This applies only where they agreed to this immediately at the writing of the tena'im.44תְּנָאִים (tena’im) lit. conditions—the agreements reached between the bride and groom’s family before betrothal. Since until that time there was no obligation on him (the father in law), it is all considered one obligation.45I.e. all the increases in the dowry are to be considered the original obligation of the father-in-law. However, if at the writing of the tena'im he obligated himself unconditionally to a set dowry, and at the time of the wedding they want to arrange to give the groom something in return for extending the time [of payment], this is prohibited. This should be done in a permissible manner.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)