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:
I would translate your sentence as:
Hélas, je ne serai pas disponible avant une date ultérieure.
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 @D. 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:
American English (even lower)
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.