Acquiring and Disposing while Trading in?

For fixed assets I know how to acquire and dispose of assets but I have a particular scenario I need assistance with.

A new fixed asset equipment was purchased from a vendor and at the same time to the same vendor, older pieces of equipment were traded in, around 1-3 items etc). The total for the new equipment is 100000 and the total for the equipment being traded in is 30000. The thing is I could easily acquire and dispose assets so that it shows I acquired it through the purchase invoice the asset for 100000 and disposed through the sales order my asset of 30000. But it wasn’t purchased for 100000. It was purchased for 70000 for it because of the difference when it was traded in. How could we accomplish this in BC?

Step by step guide would be appreciated