Commentary for Kiddushin 96:4
ובפלוגתא דהני תנאי דתניא בשכר שעשיתי עמך אינה מקודשת בשכר שאעשה עמך מקודשת ר' נתן אומר בשכר שאעשה עמך אינה מקודשת וכל שכן בשכר שעשיתי עמך
For it was taught: '[Be thou betrothed unto me] with the wage [owing to me] for the work I have done for thee, ' she is not betrothed; with 'the wage for what I will do for thee,' she is betrothed R'Nathan said: 'With the wage for what I will do for thee,' she is not betrothed; how much more so, 'with the wage [owing to me] for the work I have done for thee.' R'Judah the Prince said: In truth it was stated, whether [he declared], 'with the wage for what I have done,' or 'with the wage for what I will do for thee,' she is not betrothed; yet if he adds a consideration of his own, she is betrothed.<span class="x" onmousemove="('comment',' This proves that in R. Nathan's opinion she is not betrothed even then.');"><sup>5</sup></span>