Form With Text Box And Combo Box By Type

hello can anyone please help me with this form?

I have a form in which has one drop down combo box and one data entry text
box.

The combo drop down box comes from a query of all active contracts from my
contracts table.
the second box has a unit ID number which the user enters.

All information entered in this data entry form goes to a Unit table.

My contract table has three columns: Name of contract and Type (only three
types): A, B, C. (contract table) and then a column that says active or
inactive.
i.e

Name Type Status
113 A active
114 B inactive
115 A active
116 A inactive
120 C active
121 C inactive

This is where I am stuck…

When a user puts an unit id number and then selects from the listbox one of
the active contracts for the first time is ok. For example: the user entered
unit number 123 and selected contract: 113 which belongs to type A.
What I need is that next time the user put the same unit number in the text
box then the contracts he could choose for that selected unit number could
only any of type B or type C. he/she can no longer choose other type A.

so basically for every unit id number i could have only one contract type A, only one contract B and only one contract C. :confused:

Can someone give me some light in how to start this?