Selling Python Software

[Will Stuyvesant]

Suppose I want to sell a (small, 1000 lines) Python program.  It is a
commandline program for database applications and I have a customer. 
The customer wants to "try it out" before buying.  The try-out version
should be a full, complete, version.

As much as I like the opensource movement, I have a problem now.  If I
just give them the Python source code then they can show it to their
programmers and they have no reason anymore to pay money to me.  Sure
that would break our agreements, but you know bussiness, legal issues.
 The thing the customer is interested in is the algorithm used in the
program.  Not much I could do about such a scenario effectively.

I tried py2exe before, but someone told me it is always possible to
decompile...revealing the source code.

Anybody has a solution to this, besides more legal paperwork (I am in
europe...forget about claiming your rights here)?  Is it not possible
to give away a Windows .exe file that can not be decompiled (or only
with *extreme* difficulty)?