Code Kata: Crazy letter delivery

Imagine you live in a crazy town inhabited entirely by mathematicians. Each day a postman delivers some letters to the citizens. There are no street names in town, just house numbers. Because the traditional delivery seemed boring to the mathematicians, they invented a kind of game and talked the postman into playing it: the postman changes the original numer of house on a letter to an alternate number (read below how) and delivers the letter to the alternate house. The mathematician that lives in that house knows both the alternate and original house number from the postman. He indulges in the task of converting the original number to the alternate and only when he knows the proper conversion does he deliver the letter to the proper receiver. Example:

Original house number: 139.
Alternate house number: 126

Conversion: 139 - (1 + 3 + 9) = 126; original number minus the sum of its digits.

Another example:

Original house number: 222
Alternate house number: 8

Conversion: 2 * 2 * 2 = 8; digits of original number multiplied.

The task is to write a piece of code that helps to find the conversion, based on the original number, the alternate number and a set of rules. You can choose the rules as you like and extend them gradually. You can start with + and - operations on the original number and its digits. You can take multiplication into account, as in the second example. You can choose to apply division. You can restrict the rules to always use all of the digits or extend them to use any combination of the digits, with or without repetitions. The program may give an exact answer, or simply answer "conversion unknown", in some cases.