Install Leopard (10.5) on new (late 2009) MacMini

should i stop trying.

Yes.

(48270)

If you need 10.5 for some compatibility, you will want a Mac which shipped new prior to August 28, 2009.

See my FAQ*:

http://www.macmaps.com/usedrefurbished.html

No Mac can run a Mac operating system older than shipped with it.

- * Links to my pages may give me compensation.

Yes, it is called firmware, which can't be downgraded.

Are you sure they know how new your machine is?
This article:
http://support.apple.com/kb/HT2186

Basically indicates these restrictions for Macs of specific age:

1. Post August 28, 2009, No 10.5 installer will work on your Mac.
2. Post December 15, 2008, pre-August 28, 2009, only the prebundled Mac Mini 10.5 installer will work, and no retail installer will work for 10.5.
3. Post June 30, 2008, pre-December 15, 2008, only the retail 10.5.6 will work.
4. Post November 15, 2007, pre-June 30, 2008, the 10.5.4 and 10.5.6 retail will work.
5. Post October 26, 2007, and pre-November 15, 2007, the 10.5.1, 10.5.4, and 10.5.6 retail will work.
6. pre-October 26, 2007 all 10.5 retail will work.

Unless they checked the age, they may be sending you the wrong disc again.
The ages of various releases of 10.5 are here:

http://en.wikipedia.org/wiki/MacOS_X10.5

If they are sending you the Restore DVD that originally shipped with the Early '09 Mac mini you should be able to install 10.5 Leopard on the new Late '09 Mac mini. I used the Restore DVD from my Early '09 Mac mini to install 10.5 on two minis last Saturday; one was an Early '09 that shipped in OCT and the other was a Late '09, both with 10.6 installed. Neither the hardware nor the firmware (MM31.0081.B06) has changed on either and the Early '09 Restore DVD has all the software necessary to address the mini's hardware.

Dah•veed

The restriction on the date generally is possible to be told by serial number. You might have had an older Mac Mini that was sitting on the shelf, and by serial number was older than the 10.6 release, though prebundled with both 10.5 and an Update DVD.

I.e.: xx935xxxx is the 35th week of 2009, or starting August 27th.
xx901xxxx to xx934xxxx can use the Mac Mini installers which shipped during 2009 with 10.5. Early xx935xxxx Mac Minis may still be able to use 10.5.
xx936xxxx and later Mac Minis can only use 10.6 that was prebundled with the Mac Mini, and no 10.5.

Because it is a MacBook disc.

This is especially the case since:

http://support.apple.com/kb/HT1159

says there was no 10.5.7 nor 10.5.8 Mac Mini disc.
It either was prebundled with a 10.5.6 Mac Mini disc, or a 10.6 Mac Mini disc.

If it was prebundled with a 10.5.6 Mac Mini disc, that's the only 10.5.6 disc you can use. Otherwise you can't use 10.5.