'tambahan diskon bagi pembeli atas bea meterai' mestinya berupa bea meterai ditanggung oleh pengembang.

Kalau ini properti yang dijual di Singapura, ada dua jenis stamp duty di dalam transaksi:

1-buyer stamp duty: ditanggung pihak pembeli
2-seller stamp duty: ditanggung pihak penjual.

Frasa yang ditanyakan adalah yang pertama, alias bea meterai tanggungan pembeli.

additional discount di sini berarti sebagian bea itu diambil alih pengembang. Entah bagaimana caranya, sebab aturannya sudah jelas, yaitu persentase terhadap harga jual. Yang didiskon pastilah harganya, namun biar citra mahalnya terjaga, disiasati seolah-olah yang berkurang adalah bea meterainya.

Ref: http://ownsgproperty.com/singapore-property-stamp-duty-overv...
Buyer Stamp Duty
In Singapore, buyers must pay stamp duties when they purchase a property, unless otherwise stated on Sales & Purchase or Option to Purchase terms and conditions. ... Without stamp duty, the documents are not permissible in court as evidence of your interest in the property.

Seller Stamp Duty
Since Feb 2010, in an effort to discourage speculative flipping, the government introduced a new measure known as Seller's Stamp Duty (SSD). The latest revision was announced in 14th Jan 2011, where the Seller's stamp duty was raised up to 16% and the holding period is extended to 4 years.

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Note added at 51 mins (2013-04-24 08:43:25 GMT)
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Info tambahan:
BSD dan SSD (buyer/seller stamp duty) juga dikenal di Malaysia. Saking mahalnya, bisa dicicil.

Dari tautan berikut, Additional Buyer Stamp Duty malah merupakan sebuah frasa tersendiri. Jadi, penggalan yang tepat adalah: [Additional Buyer Stamp Duty] [discount].

Ref: http://www.iproperty.com.sg/news/taggedArticles.aspx?tagid=4...
Given the increase in the Additional Buyer’s Stamp Duty (ABSD) after the introduction of the seventh round of cooling measures, the number of foreigners purchasing private homes is likely to fall in the first half of this year, say industry insiders.

IMHO