Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for instalment; he lists it under part.
Where he writes about it:
A portion is a part viewed with reference to some one who is to receive it or some special purpose to which it is to be applied; in a restaurant one portion (i. e., the amount designed for one person) is sometimes, by special order, served to two; a share is a part to which one has or may acquire a right in connection with others; an instalment is one of a series of proportionate payments that are to be continued till the entire claim is discharged; a particle is an exceedingly small part. — from part