Here is a breakdown:

The preposition على means "provided."

When used in this sense, على must be followed by أن. The phrase على أن means "provided that," meaning, "on the condition that . . . "

The phrase على أن must be followed by a verbal phrase in the present tense (verb + subject). Example:

على أن ينقل البائع
(". . . provided that the seller transports . . . "

If the condition is negative (e.g., "provided that the seller does not transport"), the word لا must precede the verb. Example:

على أن لا ينقل البائع

The phrase أن لا is pronounced "ALLA," with the L intensified, because it is easier to merge the N with the L in speech. This should not affect the written form, but many writers do in fact merge the N with the L, so أن لا is written ألاّ, with a shadda on top of the laf-alif. I know it is confusing, but once you become familiar with it, it will be easy.

The result is that على أن لا is written على ألاّ (". . . provided that [subject] does not . . . "