וַיֹּ֣אמֶר יְהוָ֔ה סָלַ֖חְתִּי כִּדְבָרֶֽךָ׃
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Rashi on Numbers
כדברך [I HAVE FORGIVEN] ACCORDING TO THY WORD — i.e. because of what you have said, “Lest they say: ‘Because the Lord was unable, etc.’”.
Sforno on Numbers
סלחתי כדברך. As you said when I mentioned smiting them all with pestilence. Even when I said this, I had not intended to smite them all simultaneously; I had intended to let them all die, little by little, in the desert, preventing them from crossing to the land of Canaan.
Or HaChaim on Numbers
ויאמר ה׳ סלחתי כדברך, G'd said: "I have forgiven according to your word." G'd meant that He had not forgiven the sin absolutely, only, -as Moses had asked- that He would not kill them as one man. This was in response to Moses' argument that otherwise G'd would be perceived as unable to conquer the Canaanites. At any rate, He would invoke His attribute of ארך אפים by spreading the punishment over 40 years. The following verses are all illustrations of what G'd meant by invoking His attribute of ארך אפים on the one hand, and swearing an oath that all of the people would suffer retribution on the other.