A South African taxpayer can donate up to R100 000 per tax year and not have to pay any Donations Tax. Any amount over that will incur a donations tax of 20% which she will have to pay over to SARS. This would be the case if your mother was donating money to anyone else in a tax year in addition to the R100,000 gift she is giving you.
You should declare on your Tax Return (ITR12) that you received a donation under the "Amounts not considered taxable" section.
I am assuming here that your mother is a South African tax resident. If she is not, then donations tax does not apply at all on donations from non-residents to SA residents.
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