I received an email relay, this Sunday, from several rabbis passed onwards to me, eventually. (Proof that even your local rabbi has to ask a question on Mi Yodeya.)
After the metzorah (Leper) has been declared tahor, among the items he must bring to the Cohen are two birds. One is killed, and the live one is sent free to the field.
In Vayikra 14:7 the phrasing is:
ושלח את הצפור החיה על פני השדה
(Ibn Ezra explains that על means "toward")
In Vayikra 14:53, the phrasing is:
אל פני השדה
Why the difference in the word used in these two verses?