A friend bought a new bike and wanted a different crank. The shop procured and installed the new crank at no extra charge. They kept the OEM crank. The cranks were same series, just different chainring configuration. I'm only familiar with that one instance but I feel sure if you buy a different part and pay for installation the shop will give you the OEM part. If they wanted to keep it they would likely give you a price break on the alternate part. What bike are you getting and what do you want substituted?