consideration

No, the promise was made before the marriage as this letter clearly shows:

The uncle wrote,

“ 11 August, 1838. Gray’s Inn.
My dear L,

I am glad to hear of your intended marriage with Ellen Nicholl, and, as I promised to assist you at starting, I am happy to tell you that I will pay to you 150l yearly during my life time and until your annual income from your profession of a chancery barrister shall amount to 600 guineas, of which your own admission will be the only evidence that I shall receive or require.

Your ever affectionate uncle, Charles Shadwell.”

Now, the preamble to that letter says, ambiguously, this:

“Mr Shadwell was engaged to marry Ellen Nicholl (this is a binding contract). His Uncle Charles promised £150 a year in a letter after the marriage.”

In my view, the sentence construction is incorrect and, simply by rearranging a phrase, without changing any words, we arrive at this:

“Mr Shadwell was engaged to marry Ellen Nicholl (this is a binding contract). His Uncle Charles, in a letter, promised £150 a year after the marriage.” or even

“In a letter, his Uncle Charles promised £150 a year after the marriage.” or

“His Uncle Charles promised, in a letter, £150 a year after the marriage.”

In fact, you could put the phrase “in a letter” anywhere in the sentence except between “year” and “after” and you will arrive at what I believe to be the correct meaning

OK?