The Tosefta in Pesachim 2:6 is the source for selling chometz. Many Rishonim have a girsa (additional wording) at the end of the tosefta warning not to perform the sale in a way of "haramah" (trickery). It needs to be a real complete sale. It should not look like the parties don't really mean it and its just a play to make the chametz "sold" when no one is really selling it or buying it. It is not a ceremony; but rather a sale. (It is for this reason of risking appearing like trickery, that many Jews have the custom of not selling chametz to a non-Jew.)
In S.A. O"C 448:16 the Bes Yosef brings a Terumas HaDeshen who says that the chametz must be sold to a non-Jew "outside your house". The Magen Avraham and Taz explain that it means the chametz must leave your reshus. The chametz should be brought out of your home to the non-Jew's domain. This measure shows that the sale is real, as merchandise is actually changing hands.
The Bach, however wonders what people can do if they have a large amount of (example) shnapps (liquor) made of chametz. Does it all need to be transported to the non-Jew's home? So the Bach was inspired to pasken that one may grant a piece of his own domain (real estate) to the non-Jew in the sale with the chametz. That way, the merchandise did in fact enter the non-Jew's domain (without needing to transport it out). However, the Bach seriously states that one must give the non-Jew the key to the room/house that was granted to him as his domain. If he does not, then the whole sale is a trick and the chametz is not really sold.
However, the Mishnah Berurah brings the opinion of the Chemed Moshe that it is enough if the Jew guarantees access to the non-Jew, even without giving him the key. Therefore, according to that more lenient opinion, it is enough to be available to the Rabbi's phone call and let the non-Jew in when he calls. Some want the actual key and alarm code in keeping with the stricter description of the Bach. It appears that the OP's contract is in fact lenient like the M"B (C"M), unless the owner is away. Then it wants the level proposed by the Bach.