I can't even understand the obtuse title I wrote, so I'll use the real numbers below.

I had an ecredit which I used towards a new ticket. The total ticket price was $606.96, of which $533.10 was paid with an ecredit and $73.86 with cash on September 4, 2024. The ecredit had an expiration date of March 25, 2025.

Today I saw that the fare had gone down to $516.96, so I call Delta to get it refared which they do, and I am given a $90 ecredit. That ecredit presumably is made up of returning my $73.86 cash payment for the old ticket and $16.14 of the March 25, 2025 expiring ecredit.

The new ecredit has an expiration date of march 25, 2025, even though the bulk of that ecredit is the 73.86 that I paid in cash on September 4th.

In other words, the $73.86 in cash that I paid on September 4th is now part of an ecredit expiring on March 25, 2025.

Is that how they are supposed to do it? Is this consistent with the fare rules? Shouldn't I get a year to use the ecredit from the cash payment on September 4th?