יברכך ה’ וישמרך יאר ה’ פניו אליך ויחנך (נשא ו:כד-כה)

It says in the Medrash (יא: ה-ו) that “וישמרך’” – ישמר לך את הקץ – “ויחנך” – במתנת חנם” – He will save your reward for  the end. ויחנך” – במתנת חנם – with an unearned reward..

It can be explained with a story that the Gemara relates, (תענית כה:א): The holy Tanna Rebbe Chanina ben Dosa lived in great poverty.  His wife once asked: “How much longer do we need to struggle?

Reb Chanina replied, “Let’s daven to Hashem. He should send us something.”

When they davened a hand came down to give them a gold leg of a table.  His wife then had a dream where she saw all tzaddikim sitting, לעתיד לבא – when Moshiach will come, at golden tables with three legs , and she and her husband are seated at  a table with only two legs. 

So they davened to Hashem that they don’t need the gold leg and it was taken back from them.  From this story we learn that when Hashem gives us good in this world it gets deducted from future merits לעתיד לבא.

Hence when the Kohanim bless the Yidden with myriads  of great  brochos, these blessings  could diminish our blessings לעתיד לבא, so the Kohanim bless: “וישמרך’” – ישמר לך את הקץ – Hashem should watch over our future rewards לעתיד לבא, that even though He bestows upon us lots of bracha now, nothing should be diminished from later – all the brochos that we receive on this world should be “ויחנך” – במתנת חנם – as the Medrash says – “A free gift!”