sechora with peiros sheviis

Yesh lachkor whether the issur of sechora with peiros sheviis stems from the fact that if one uses the peiros for barter one cannot fulfill the mitzvah of achilas peiros, or whether sechora is an independent issur? 

Ramban counts the mitzvah of achilas peiros sheviis as a mitzvas aseh, and cites the derasha of l'achla v'lo l'sechora as his source, implying that the problem with sechora is that it negates the possibility of fulfilling the mitzvah of achila.  Rambam never counts eating peiros as its own mitzvah, so sechora must be an indepdent issur.

 

The Steipler suggests that this question may underpin a sasfeik in the gemara.  (Bechoros 12b) discusses whether one can use an animal purchased with peiros sheviis to be podeh a peter chamor. 

 

לפדות בבהמת שביעית ודאי לא תיבעי לך לאכילה אמר רחמנא ולא לסחורה כי תיבעי לך ספק

 

Rashi explains:

 

ודאי לא תיבעי לך. פטר חמור ודאי לא תיבעי לך דכיון דבעי למיתב פדיונו לכהן לא פריק ליה דלאכלה אמר רחמנא ולא לסחורה ונמצא זה קונה חמור בפירות שביעית שאינו ראוי לאכילה: ספק. כגון שילדה אמו זכר ונקבה דאמר במתני' מפריש טלה והוא לעצמו:

 

In a case of safeik peter chamor you have to be mafrish a lamb to remove the issur hanaah from the chamor, but you don't have to give it to the kohen because ha'motzi mei'chaveiro alav ha'raaya.  The pediya = sechora, but the lamb can still be still eaten by the owner.

If sechora is a problem because it negates the mitzvah of achila, in this case there should be no issue because the owner can eat it.  But if sechora is an issur in its own right, then this case should pose a problem.