Neither English, nor French native but I believe that hélas is the counterpart of “alas”. wiktionary says

From Old French elas, variant of a las, from a (“ah”) + las, from
Latin lassus (“weary”).

I believe thus that English “alas” and French hélas are like brother and sister.
For further examples see here:

http://www.cnrtl.fr/definition/h%C3%A9las

I would translate your sentence as:

Hélas, je ne serai pas disponible avant une date ultérieure.

EDIT

1) Based on @aCOSwt’s comment:

They (i.e. alas and hélas) are not exactly as brother and sister. Alas cannot be used as a noun while hélas can be used as a noun (since the XVth century).

2) Based on wp_update-1694464596. Ben Knoble’s comment:

Hélas seems in more current usage in France than alas does in (American) English.

3) Extension of (2); comparison of alas and hélas frequency:

https://books.google.com/ngrams/graph?content=alas&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Calas%3B%2Cc0

American English (even lower)

https://books.google.com/ngrams/graph?content=alas&year_start=1800&year_end=2000&corpus=17&smoothing=3&share=&direct_url=t1%3B%2Calas%3B%2Cc0

French

https://books.google.com/ngrams/graph?content=h%C3%A9las&year_start=1800&year_end=2000&corpus=19&smoothing=3&share=&direct_url=t1%3B%2Ch%C3%A9las%3B%2Cc0

EDIT 2

In the Greek language there is the world pheû. It means hélas. It would be interesting to see any connection between the etymology of hélas and the pheû.